Object reference not set to an instance of an object

Question

When I try to open the page from my IDE in VS 2008 using "VIEW IN BROWSER" option I get "Object reference not set to an instance of an object" error.

The piece of code I get this error :

 XResult = Request.QueryString["res"];    
 TextBox1.Text = XResult.ToString();

Measurable

Answer 1

The problem here is that XResult is null and when you call ToString on it the code produces a NullReferenceException. You need to account for this by doing an explicit null check

TextBox1.Text = XResult == null ? String.empty : XResult.ToString();

Epifocal

Answer 2

If you are opening the page without the "res" query string then you need to include a check for null before you do anything with it.

if (Request.QueryString["res"] != null)
{
    XResult = Request.QueryString["res"];
    TextBox1.Text = XResult.ToString();
}

Gaberones

Answer 3

That error could be Because the REquest.QueryString method did not find a value for "res" in the url so when you try to do the "toString" to a null object whrow that exeption.

Cohort

Answer 4

Your code is expecting a query string http://localhost:xxxx/yourapp?res=yourval. It's not present in the address supplied to the browser. In the web section of your project properties, you can supply an appropriate URL. Of course, shoring up your code to allow for this would be advisable.

Muriah

Answer 5

XResult is already a string, so calling ToString isn't necessary. That should also fix your problem.

Dipper

.ToString() on a string won't throw an error. The issue is that XResult is null because it's not finding "res" in the query string. – Bernete

It is already a string and the call to .ToString() is not necessary, but it will not fix your problem. – Gaberones

It will solve the null reference exception, because calling .ToString() on a null will throw the exception. – Dipper

Answer 6

The problem here is that XResult is null, and when you call ToString on it the code produces a NullReferenceException. You need to account for this by doing an explicit null check:

if (Request.QueryString["res"] != null)
{
    XResult = Request.QueryString["res"];
    TextBox1.Text = XResult.ToString();
}

Boiled